1. Industrial Field of Utilization
The present invention relates to spark plugs to be used in internal-combustion engines for automobiles.
2. Description of the related Art
In conventional gasoline internal-combustion engines for automobiles, a method for supplying a very rich air-fuel mixture into combustion chambers has been used as one means for ensuring specified engine operation performance at low temperatures. This method, however, has the side effect of producing much carbon in the combustion chamber, due to the incomplete combustion of the air-fuel mixture. This carbon is gradually deposited on the surface of an insulator of the spark plug installed in the combustion chamber. Such a spark plug covered with carbon residue has a tendency that the high voltage supplied from an ignition coil leaks through the carbon residue on the insulator, resulting in sporadic sparks jumping across the spark-plug gap and accordingly increased combustion misses and misfiring of the air-fuel mixture in the combustion the combustion chamber. Particularly in a four-wheeled automobile equipped with batteries for ignition, unlike the capacitor-discharge ignition (CDI) system which improves leakage characteristics as seen in a two-wheeled motor vehicle, a problem is likely to occur in connection with the deposits of soot and carbon residue.
To solve this problem, a special spark plug for internal-combustion engines is disclosed in Japanese Patent Publication No. 58-40831. This spark plug is designed such that a spark jumps across a first spark-plug gap under a normal operating condition when no carbon residue is present over the end of the insulator nose and the inside bore wall surface are stained with soot and carbon deposits. This spark reaching the second gap burns off carbon deposits covering the inside bore wall surface, recovering the normal surface condition for sparking.
However, since the second gap of the above-described conventional spark plug for the internal-combustion chamber is provided on the wall surface side of the combustion chamber relative to the first gap, the core of flame produced by a spark across the second gap can not easily contact the mixture. In addition the second gap is smaller in size than the first gap, and therefore the flame core can not grow larger. Consequently, there occurs the problem that a spark generated across the second gap results in unreliable ignition of the air-fuel mixture.
For the purpose of solving this problem, spark plugs for internal-combustion engines have previously been proposed by the present inventor et al in Japanese Patent Application No. 63-83369 (filed Apr. 5th, 1988).
This spark plug for the internal-engine is designed to ionize the ambient atmosphere by capacity discharge, and to burn off carbon deposits in the inside bore of the insulator by utilizing induction discharge generated within the most ionized area of the range of ionization. In this spark plug for the internal-combustion engine, because the small-diameter portion of the center electrode projects out of the inside bore of the insulator over the end of the insulator nose a larger flame core can be produced by the gap, thereby enabling an improvement in the ignition of the air-fuel mixture.
The automotive industry will be more and more demanded in future to build automobiles of higher performance and accordingly will require component parts of better quality. Since spark plugs for internal-combustion engines in particular directly determine the engine operation performance, it can well be expected that much stricter requirements will be imposed for the manufacture of spark plugs having an improved carbon burn-off effect.
It is, therefore, an object of this invention to provide spark plugs for the internal-combustion engine which are able to remarkably improve the ignition of the air-fuel mixture and the carbon burn-off effect to remove carbon deposits from the insulator.